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Phase Control of Eu3+-Doped YPO4 Nano/microcrystals Peng Li, Yanpeng Zhang, Lei Zhang, Feng Li, Yaxin Guo, Yanghui Li, and Weiping Gao Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01038 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 24, 2017

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Crystal Growth & Design

Phase Control of Eu3+-Doped YPO4 Nano/microcrystals Peng Li, a Yanpeng Zhang, *, a Lei Zhang, a Feng Li, a Yaxin Guo, b Yanghui Li, c Weiping Gao d a

Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Lab of Information Photonic Technique, Xi’an Jiaotong University, Xi’an 710049, China

b

Electronic Materials Research Laboratory, Key Laboratory of Ministry of Education & International Center for Dielectric Research, Xi'an Jiaotong University, Xi'an 710049, China c

Department of Chemistry, South University of Science and Technology of China, Shen Zhen 518000, China

d

School of Physics and Optoelectronic Engineering, Xidian University, Xi'an 710071, China

ABSTRACT: The crystal phase control is one of the most important issues in materials science, because different atomic arrangements and electronic structures of crystals will endow materials with enhanced or new functionalities. However, exploring a general rule to the phase control is extremely difficult, since the formation of new crystal phases are usually induced by many reaction parameters or methods. Here we describe a Eu3+ (5 at. %)-doped YPO4 nano-/microcrystal system in which the phase can be rationally tuned in hexagonal or tetragonal. We confirm that manipulating the relative concentrations of phosphate groups and free RE3+ ions in using different approaches under reaction solution, can determine the formation of final crystallographic phase of the Eu3+-doped YPO4 nano-/microcrystals. Experimental observation show crystal structure offering powerful control over the morphologies and optical emission properties of the resulting nano-/microcrystals. We believe that the structural control scheme, demonstrated here in Eu3+-doped YPO4 nano/microcrystals, could be extended to other inorganic nano-/microcrystal systems for their desirable applications and their correlated fundamental research.

Introduction The crystal structure of YPO4 is known to mainly exist in two polymorphic forms, namely, hexagonal (h-) and tetragonal (t-) phases.1,2 The former phase was firstly reported in 1989. It has P6222 space group (No. 180) with a rhabdophane-type structure (a = 0.6833 nm and c = 0.6291 nm) where Y3+ ions occupy a D2 point-group symmetry site and have an eight coordination environment formed by oxygen ions (four at 0.2578 nm and the other four at 0.2257 nm).3 The later one has I41/amd space group (No. 141), where Y3+ ions occupy a D2d point-group and have two kinds of coordination bonds (one in 0.2309 nm and the other in 0.2381 nm).4 Till now, a lot of efforts have been devoted to the wet chemical synthesis of YPO4 nano-/microcrystals with the above mentioned two phases.5-14 The crystals, doped with Ln3+ ions and adopting the tetragonal phase, hold the advantages of sharp emission spectrum, large Stokes shift, long life time, high chemical/thermal stability, reduced photobleaching, and low toxicity, have been used for a wealth of applications, such as optoelectronics, field-effect transistors, clean energy, new types of sensors, and biomedicine research.15-18 In contrast to tetragonal structure, the Ln3+-doped hexagonal phase crystals were not regarded as promising luminescence materials, due to its metastable nature and the trapped O-H luminescence quencher.19,20 However, tunable emission in a large color gamut on the basis of codoped hexagonal structure YPO4 has been reported recently, which opened a window for their potential use in

the fields such as solid state lighting and field emission displays.21 Considering that these applications stem from their structural diversity and the properties of the materials closely interrelate with their crystal structure, the controllable synthesis of inorganic materials in a desirable phase remains attractive but challenging.22-26 For instance, Yan and co-workers have demonstrated that EDTA facilitates the formation of metastable tetragonal phase REVO4 (RE = La, Ce), and the morphologies can be tunabled.27,28 They explained the preferred formation of t-LaVO4 based on the lower coordination number (8, versus 9 for monoclinic phase LaVO4). In most cases, this mechanism was found to be appropriate and also carried out to explain the phase formation for other REVO4 hosts, such as YVO4 and GdVO4.29-31 Similarly, some experimental studies demonstrated that for the Ln3+ ion doped YPO4 system prepared in hydrothermal conditions, the phase transformation from t- to h-phase YPO4 is easy to be controlled or fine-tuned.26 Differently, Y3+ have the same coordination number of 8 in both the t- and h-phase YPO4,1,2 which drives us to understand the phase transformation from t- to h-phase YPO4 without the conception in terms of coordination number, strong steric hindrance, and repulsion. To date, the mechanism of the phase transformation from t- to h-phase YPO4 crystal remains unclear. Nigthoujam’s and other groups reported that after annealing at high temperature (700 - 900 oC), the crystal

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structures can be changed from hydrated h-phase (like hphase YPO4) to dehydrated t- (like t-phase YPO4) or mphase, accompanying with greatly enhanced emission intensities.19,20 Accordingly, the phase transformation mechanism has been demonstrated via diffusioncontrolled solid-state transformation.32 Even though the annealing treatment is an effective method to enhance the fluorescent intensity, there are some inherent limitations with regard to the user safety, energy cost, contamination of impurities, and the difficult investigation of the mechanisms underlying the phase and shape. In addition, due to particle aggregation after annealing, it is especially unsuitable for the synthesis of high-quality and uniform crystals. Therefore, a new idea or facile technique is desired to accomplish the phase transformation from h- to t-phase YPO4 in an efficient way. All in all, despite numerous reports regarding the synthesis studies of lanthanide orthophosphate materials, a comprehensive understanding of the phase formation/transformation on the Ln3+-doped YPO4 system is still-not achieved. To achieve a control over the phase structure of nano-/microcrystals, it is necessary to know clearly the effects of reaction conditions on the target product, especially whether there exist common factors or general rules in all reaction systems that encourage the phase formation/transformation in Ln3+-doped YPO4. In fact, the relative concentration of phosphate groups (PO43-) and free RE3+ ions in reaction solution is a key factor but often neglected or missed in the study of phase and shape control of nano-/microcrystals, since it is always involved in too many experimental parameters. As is known, trivalent Eu ions are more sensitive to the site symmetry and its surrounding crystal-field (CF) of the host material than other Ln3+ ions. A slight variation of local structure will bring about a significant change of the optical properties,33-37 which, in turn, may facilitate to determine the local structures change of Eu3+ ions embedded in nano-/microcrystals. Enlightened by the characteristics of YPO4 host and Eu3+ ion described above, Eu3+-doped YPO4 is well selected as it has two typical phase to realize the phase/structure-controllable synthesis in this study. In our experiments, we adopted 5 at. % Eu3+ ions (relatively low concentration) doping into host lattice so as to preserve the intrinsic structure of the YPO4 crystals and do not induce the significant concentration quenching of luminescence.38,39 At first, we prepared hphase Eu3+-doped YPO4 crystal via hydrothermal method using trisodium citrate (Cit3-) as chelating ligand. The phase transformation process from h- to t-phase Eu3+doped crystals can be well observed by simply tuning the molar ratio of PO43- : RE3+ : Cit3- under hydrothermal conditions. Secondly, to achieve a better understanding of the phase formation and gradual transformation process, various parameter-controlled synthetic experiments, including the concentration of phosphate group, the type and the concentration of chelating ligand, the adjustment of pH values, were investigated in detail. Thirdly, based on a series of contrast experiments, the influence of the relative concentrations of PO43- and free RE3+ ions on the

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phase formation/transformation in Eu3+-doped YPO4 crystals can be definitively identified. Finally, we also explain that the corresponding changes in morphology and luminescence property are dependent on crystal structures. Moreover, a better understanding of the phase formation/transformation based on this study will direct us to produce other inorganic materials with predesigned crystal structure and their correlated multifunctional properties. Experimental Section Materials. The rare earth oxides RE2O3 (RE = Y, Eu) (99.999%), ammonium dihydrogen phosphate (NH4H2PO4), trisodium citrate (labeled as Cit3-), isopropyl alcohol (IPA), ethylenediamine tetraacetic acid (EDTA), and ethylenediamine (EN) were purchased from Sinopharm Chemical Reagent (Shanghai, China). All chemicals were analytical grade reagents and used without further purification. Distilled water was used throughout the experiment. Synthesis of Eu3+ (5 at. %)-doped YPO4 samples (written as YPO4:Eu) with different crystal phases using the trisodium citrate (Cit3-) as chelating ligand. By dissolving the Y2O3 and Eu2O3 in HNO3 solution with agitation under heating, the Y0.95Eu0.05(NO3)3 aqueous solution (2 M) was obtained. Then, 10 mL trisodium citrate (Cit3-) solution (0.2 M) was added dropwise to 5 mL of Y0.95Eu0.05(NO3)3 solution (0.4 M) to form a clear complex of the RE3+ (Y3+, Eu3+)–Cit3-. After vigorous stirring for 30 min, 10 mL of NH4H2PO4 solution (0.2 M) was mixed into the above-mentioned solution under continuous stirring. Using the addition of HNO3 solution, we adjusted the pH value to be about 1. Under vigorous stirring for 1 h, we poured the resulting suspension to the Teflon-lined stainless steel autoclaves and heated it for 9 h at 180 oC. After being cooled to room temperature, to collect the final h-phase YPO4:Eu (1:1:1 molar ratio for PO43- : RE3+ : Cit3-) microrods we washed the white precipitates with distilled water and absolute ethanol by centrifugation, and then dried it for 12 h at 80 oC. All the other samples were prepared with a similar method except for changing the molar ratio of PO43- : RE3+ : Cit3- from 1:1:1 to 13:1:1. It is to be noted that we have checked the pH value to be kept in 1 before each starting reaction. Synthesis of YPO4:Eu samples with different crystal phases by changing the type and the concentration of chelating ligand. The same procedure was employed while preparing, expect that IPA, EDTA, and EN as chelating ligand were added respectively instead of the Cit3-. The molar ratio of PO43- : RE3+ : IPA/EDTA/EN was adjusted to a suitable range in the respective reaction system. Synthesis of YPO4:Eu samples with different crystal phases by adjusting the pH values in the absence of any capping agents. The same procedure was employed while preparing, except that no chelating ligand was used in the reaction system and the pH value of the precursor solution was adjusted to 0.5, 1, 3, 5, and 7, respectively.


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Characterization. The crystal structures and phase compositions of the samples were identified by X-ray powder diffraction analysis using Rigaku D/Max2550 with Cu Kα1 (λ=1.5406 Å) radiation under 40 kV, 50 mA. The scanning rate was 8°/min in the two-theta ranging from 10° to 60°. The scanning electron microscopy (SEM), transmission electron microscopic (TEM) and selected area electron diffraction (SAED) images were obtained by Hitachi S-4800 at an accelerating voltage of 3 kV and JEM-2010 at an acceleration voltage of 200 kV, respectively. All the luminescent spectra and decay lifetimes were performed on Hitachi F-4600 fluorescence spectrophotometer using 150 W xenon lamp as excitation source. All measurements were recorded at room temperature. Results and Discussion XRD Study. Figure 1a shows the XRD patterns of the YPO4:Eu samples prepared at different molar ratios of PO43- : RE3+ : Cit3- ranging from 1:1:1 to 13:1:1. For the samples achieved at 1:1:1 and 3:1:1, h-phase can be obtained. The h-phase is in hydrated YPO4·0.8H2O form (JCPDS card No. 42-0082). No peaks of any other impurities are detected, and the diffraction peaks are comparatively sharp, meaning that the samples have good crystallinity. When the molar ratios are increased to greater range from 5:1:1 to 9:1:1, two phases, i.e. h- and t-phases (JCPDS card No. 11-0254), coexist and the new t-phase dominates on a large scale in addition to the h-phase (marked with symbol “h”). At higher molar ratios (higher than 11:1:1), intensity of tetragonal peaks further increases and hphase is completely converted to t-phase. Obviously, the process of increasing the PO43- concentration can put forward the formation of t-phase YPO4:Eu, in which phase transformation takes place and does influence luminescence intensity (as shown later). In addition, altering the Cit3- concentration can also result in the presence of the similar phase transformation. Figure 1b shows the XRD patterns of the products obtained at different molar ratios of PO43- : RE3+ : Cit3- while keeping a lower Cit3- concentration in initial stage, only a half. It is clear and prominent that 3:1:0.5 leads to the formation of a t-phase YPO4:Eu, above which, the h-phase transforms a t-phase YPO4:Eu totally. In our case, the Cit3concentration takes responsibility for the tendency of phase transformation. The less Cit3- that is added while keeping the PO43- concentration constant, the faster the new t-phase appears.

Figure 1. XRD patterns of YPO4:Eu samples prepared at different molar ratios of PO43- : RE3+ : Cit3-. (“h” stands for hexagonal phase YPO4:Eu) In order to shed light on this faster phase transformation process, the functions of chelating ligand in reaction solution 3should be explained here. In fact, taking the Cit as an example, there are double roles: (i) it can alter the free energies of different crystalline facets through their interaction. This alteration may significantly affect the relative growth rates of the crystalline facets. (ii) it is a kind of organic ligand having four binding sites, including one hydroxyl group (OH ) and 3+ 3three carboxylate groups (COO ), which can form RE -Cit complexes by means of stronger coordination interaction 3+ with RE ions. According to LaMer’s model, such complexes 3+ could control the free RE concentration in solution, and thus help to change the nucleation and growth of the crystals 40 in the view of dynamic process. Since the final crystal structures are determined in the both stage, initial nucleation and 41 further growth, we deduced that the concentration of free 3+ 3+ 3RE ion controlled by such RE -Cit complexes in solution, is crucially important for driving the phase formation. Due to 3+ 33+ the formation of the RE -Cit complexes, most of the RE ions cannot exist in the form of free ions, which can be firmly confirmed by the fact that the system still remains as a clear solution even after NH4H2PO4 is added. The decrease in con3centration of Cit in our second experiment (Figure 1b) actu3+ ally lead to more free RE ions in initial solution, and in this 3case a small number of PO4 groups are just needed to guide the new t-phase formation. This conjecture is supported strongly by our results obtained in IPA-dependent experiments. As shown in Figure 2, the rapid phase transformation is successfully controlled by 3changing the PO4 concentration added to the IPAdependent reaction. In which, the YPO4:Eu sample prepared 33+ in 1:1:1 of PO4 : RE : IPA, and even in 0.5:1:1, also show a well-crystalline tetragonal system with extra h-phase diffrac33+ tion peaks, while the sample prepared in 1:1:1 of PO4 : RE : 3Cit show a complete h-phase. However, these extra h-phase peaks are weak as compared to those of the t-phase. With 3increase in the PO4 concentration, the fully phase transformation from h-/t-phase mixture to pure t-phase occurs for 33+ 3:1:1, much earlier than that finished in 11:1:1 of PO4 :RE : 33Cit . IPA have the similar function of chelating agent as Cit .



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Figure 2. XRD patterns of YPO4:Eu samples prepared at 33+ different molar ratios of PO4 : RE : IPA. (“h” stands for hexagonal phase YPO4:Eu) -

Differently, IPA molecule only has one hydroxyl group (OH ) 3to participate the chelating reaction (Figure 3), whereas Cit has four binding sites discussed above, all of which will participate the chelating reaction (Figure 3).The distinctions of 3+ the chelating ability with RE will result in the different con3+ centration of free RE ions. Because the weaker interaction 3+ 3between RE and IPA compared to Cit , the concentration 3+ of free RE ions is increased too much in initial IPA3dependent solution and thus less PO4 concentration is required to accomplish the so fast phase transformation. Therefore, this result is consistent with the phase formation rule as proposed above that depend on both the concentra33+ tions, the PO4 and the free RE ions.

Figure 3. Structures of chelating agents and RE complexes. The similar trend is also observed in a parallel experiment, in which EDTA and EN are respectively introduced into the reaction system (Figure 4). The result from Figure 4a reveals that the sample keeps the pure hexagonal phase in 1:1:1 of the 33+ molar ratio of PO4 : RE : EDTA. At 3:1:1, the peaks of tphase YPO4:Eu appeared, while h-phase diminished prominently. As the molar ratio reached 7:1:1, the hexagonal phase completely disappeared. Hence, the h- to t-phase transformation occurs at 3:1:1 and finishes at 7:1:1. As the molar ratio rise higher than 7:1:1, the samples sustain the tetragonal structure. EDTA molecule has six binding sites in which

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Figure 4. XRD patterns of YPO4:Eu samples prepared at 33+ 3different molar ratios of (a) PO4 : RE : EDTA and (b) PO4 3+ : RE : EN. (“h” stands for hexagonal phase YPO4:Eu) two single pairs of electrons on the nitrogen atom and all four carboxylic groups (COO ) will participate the chelating reaction (Figure 3). In contrast, EN molecule only has two single pairs of electrons on the nitrogen atom to participate the chelating reaction (Figure 3). Deduced from the phase 3formation rule we discussed earlier, the amount of PO4 group required for driving the phase transition in each reaction system, should be quite different. The t-phase emergence and the entire transformation from h-phase to t-phase in the EN-dependent reaction should be faster than that in EDTA-dependent reaction. In case of YPO4:Eu sample pre33+ pared in 1:1:1 of PO4 : RE : EN, pure h-phase YPO4 was not obtained and, instead, a mixture of the h- and t-phases YPO4 can be indexed by the XRD pattern (Figure 4b). With in3crease in PO4 concentration, the transformation to pure tphase from the mixture phases in these samples is evident. Notably, the phase conversion is complete for only 5:1:1, less than the 7:1:1 required for a complete phase transformation in EDTA-dependent experiments. Thus, our hypothesis is consistent with our EN-dependent experimental observations. On the basis of our experimental results and analysis above, it is concluded that the phase formation/transformation for YPO4 crystals are determined by 33+ the concentrations of PO4 and free RE ions in reaction solution. The h-phase YPO4 crystals are favored in a low con3+ 3centration of PO4 and free RE ions, and increasing one of them in using different approaches can lead to a formation of t-phase YPO4 (Figure 5). However, the exact calculation in these concentrations required for preparing desirable phased YPO4 are limited at the present time, since it is associated with many reaction parameters, for example, concentration of precursor, pH value, ionized constant of NH4H2PO4, che3+ lating constant of chelating agent with RE ions, etc.


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Figure 5. Schematic illustration of the phase formation/transformation mechanism of YPO4:Eu by increasing 33+ the concentration of PO4 or free RE ions in using different approaches. In order to test the phase formation/transformation rule in general, we carried out the synthesis of YPO4:Eu crystals at different pH values without any chelating agent. From Figure

SEM Study. The typical scanning electron microscopy (SEM) images for YPO4:Eu samples with tunable shapes and 33+ 3size prepared at different molar ratios of PO4 : RE : Cit are shown in Figure 7. At normal molar ratio of 1:1:1, the SEM image in Figure 7a reveals that YPO4:Eu crystal consists of uniform hexagonal submicroprisms with 10 μm in length, 10 μm in diameter. When the molar ratio is 3:1:1, the regular hexagonal microprisms shaped crystals with an average size of 15 μm in length and 5 μm in diameter are observed from 3Figure 7b. The results show that increasing the PO4 concentration has little effect on the microprism morphology, but significantly results in an increase in the lengths and aspect 3ratios. According to Gibbs-Thompson theory, a higher PO4 concentration actually encourage a higher monomer concentration, and thus accelerate 1D structural growth and benefit the formation of longer microcrystals. Further increase of the molar ratios to greater range from 5:1:1 to 9:1:1 (Figures 7c-e), the obtained samples show two distinct morphologies that include microprisms and microspheres, well consistent with the presence

Figure 6. XRD patterns of YPO4:Eu samples prepared at different pH values without any chelating agent. (“t” stands for tetragonal phase YPO4:Eu) 6, we found that the pH value plays an important role in structure formation of YPO4:Eu crystal, the increase in initial pH value can lead to a significant phase transformation from h-phase YPO4 to t-phase one. We could explain this transformation based on the role of the pH values in the NH4H2PO4 ionized process. The ionized procedures of NH4H2PO4 in the reaction can be shown by the following three equations: +

NH4H2PO4 ⇌ NH4 + H2PO4 -

+

2-

+

H2PO4 ⇌ H + HPO4 HPO4 ⇌ H + PO4

-

(1)

2-

(2)

3-

(3) +

The reaction system will contains many H ions at the beginning when the initial pH value is low. As a result, it must restrain the ionized process of NH4H2PO4, H2PO4 and 2HPO4 according to the chemical reaction balance law. Then, the eq (1-3) will have difficulty proceeding, easily lead3ing to a low concentration of PO4 in reaction environment. On the basis of the phase formation rule as we discussed 3above, the low concentration of PO4 is beneficial to the formation of the h-phase YPO4:Eu. In case of relatively high + pH value conditions, fewer H ions will encourage the reaction equilibrium in the positive direction, resulting in the 3high concentration of PO4 . The phase formation/transformation rule is also consistent with our pHdependent experimental observations showing that an in3crease of the concentration of PO4 induced by the initial pH value favors the formation of t-phase YPO4:Eu.

Figure 7. Morphologies for YPO4:Eu samples prepared with 33+ 3different molar ratios of PO4 : RE : Cit . (a) 1:1:1, (b) 3:1:1, (c, cl) 5:1:1, (d) 7:1:1, (e) 9:1:1, (f) 11:1:1, (g) 13:1:1.



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Table 1 Summary of the experiment conditions and the corresponding morphologies and dimensions as well as phases of the samples 3-

3+

3-

samples

PO4 : RE : Cit (molar ratio)

crystal phase

morphology

diameter

length

a

1:1:1

hexagonal

submicroprisms

10 μm

10 μm

b

3:1:1

hexagonal

microprisms

5 μm

15 μm

c

5:1:1

hexagonal & tetragonal

microprisms & microspheres

d

7:1:1



e

9:1:1



f

11:1:1

tetragonal

nanosheets

100 nm

g

13:1:1

tetragonal

nanosheets

100 nm

of two phases observed by XRD patterns (Figure 1a). However, the dominant morphology of the products is microsphere with an average diameter of 5 μm. The magniﬁed SEM image of those microspheres depicted in Figure cl shows that porous spherical structures were formed by the self-assembly of nanosheets with thicknesses ranging from 20 to 40 nm. Such a self-assembly process could be driven by the interaction 42 between inorganic materials and chelating ligands. XRD patterns (Figure 1a) and SEM images (Figures 7a and b) analysis reveal that the microprisms are h-phase YPO4. We therefore concluded the microspheres and these nanosheets to be t-phase YPO4 and we further confirmed this by electron diffraction study (as shown later). In case of high molar ratios (11:1:1-13:1:1), the complete conversion of h- to t-phase occurred, resulting in formation of only irregular shaped and dispersive nanosheets with diameters of about 100 nm (Figures 7f and g). The decreased size to about 100 nm nanosheets from about 5 μm microspheres for the t-phase YPO4 crystals can be explained based on the critical radius of nuclei and their nucleation rate. The critical radius of nuclei, 43 rc, is expressed as rc = 2γVm/RT ln S.

sponding experimental conditions are given in Table S1, S2, S3 and S4 (see the Supporting Information). TEM Study. Figure 8a shows the transmission electron microscopy (TEM) image of a section of a single submicro33+ prism prepared at 1:1:1 for reaction molar ratio of PO4 : RE : 3Cit , reflecting its hexagonal profile. The selected area electron diffraction (SAED) image of the sample is shown in Figure 8b. Clearly, hexagons with equidistant dots from the zone axis can be constructed from the SAED pattern. For a given hexagon, an angle between any two adjacent dots

Eq. 1

Where γ is the surface free energy per unit area (γ > 0), and Vm is the molar volume of bulk crystal. Saturation ratio of the reaction solution, S, is closely tied with the concentration of 3PO4 . The 11:1:1, and even higher molar ratios, providing ex3tremely high concentration of PO4 , easily lead to an increase of S and then lead to a decrease of rc. Smaller rc of nuclei with a short nucleation burst give rise to smaller size of a large number of crystallites, which results in a decrease in final crystal size. The corresponding morphologies and dimensions as well as phases of the products obtained at 33+ 3different molar ratios of PO4 : RE : Cit are summarized in Table 1. Similar trend are also observed in the case of samples pre3pared in the Cit -reduced, IPA-, EDTA- and EN-dependent 3experiments instead of Cit -dependent experiments. The simultaneous phases, morphologies and dimensions evolution of the corresponding products are shown in Figure S1, S2, S3 and S4, respectively and the summaries of the corre-


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Figure 8. TEM images and SAED patterns of YPO4:Eu sam33+ 3ples prepared with different molar ratios of PO4 : RE : Cit . (a, b) 1:1:1, (c, d) 5:1:1, (e, f) 13:1:1. through the zone axis is 60°. All the spots in the SAED pattern have been assigned to different h-phase planes along with [0001] zone axis. This study establishes the formation of single crystalline YPO4:Eu crystals with h-phase. Figures 8c and d show the TEM images and the corresponding SAED patterns of a microprism and a self-assembled microsphere obtained at 5:1:1. XRD pattern has shown a mixed phase system for this. Herein, the SAED analysis respectively confirmed that the microprisms are h-phase YPO4:Eu, and the microspheres are t-phase YPO4:Eu. For the samples prepared at 13:1:1, the TEM image (Figure 8e) and SAED pattern (Figure 8f) exhibit the nanosheets have an average diameter of 100 nm, and a clear three-ring pattern showing (200), (112), and (312) planes of the t-phase YPO4:Eu nanocrystals, respectively.

bond lengths (R1=0.2257 nm, R2=0.2578 nm) is big, whereas the difference in the t-phase YPO4:Eu (R1=0.2309 nm, 1 R2=0.2381 nm) is rather small. The big difference in two 3+ 2− Eu –O chemical bond lengths in the h-phase YPO4:Eu could lead to a longer wavelength distance between the two corresponding CTB position as compared with that in tphase YPO4:Eu, and thus they would be combined into a 3+ 2− broader CTB. In addition to this, the average Eu –O bond (R=0.2418 nm) distance in h-phase YPO4:Eu is relatively longer than that in t-phase YPO4:Eu (R=0.2345 nm), and as a consequence, a redshift of the CTB band was observed. By a comparison of all the excitation spectra, overall absorption intensity is increased along with the phase transformation to t-phase because of the removal of water molecules, water molecules are observed or confined within h-phase YPO4:Eu leading to high non-radiative rate.

Why does h-phase YPO4 have a prism-like morphology, whereas t-phase YPO4 consist of microspheres or nanosheets? This could be associated with the change of crystal structure. The crystalline structure of nuclei is critical for directing the intrinsic morphologies of crystals because of 44 its characteristic symmetry. According to previous study, the hexagonal structure is well-known to exhibit anisotropic character, which means a higher growth rate along the c axis 45 and a lower one perpendicular to the c axis. The hexagonal YPO4 crystals grow preferentially along the [0001] direction through oriented attachment process in which preferential adsorption of special crystallographic facets endow them 46 with regular prism-like profile. On the contrary, tetragonal YPO4 has no preferred growth direction in the crystalline 47 phase. No anisotropic nature in the t-phase YPO4 always presents sphere, sheet or particle morphology instead of microprisms or microrods. Considering the close correlation between crystal structure and morphology, the phase control is powerful in tuning the morphology of micro/nanocrystals. The intrinsic shape will be determined, so long as the phase of YPO4 is well controlled. Photoluminescence study. Excitation study. Figure 9a shows excitation spectra of the YPO4:Eu samples as-prepared with different molar ratios 33+ 3of PO4 : RE : Cit . Excitation spectra are recorded by monitoring the emission wavelength at 595 nm. The excitation spectra consist of broadband between 230-280 nm assigned to the Eu-O charge transfer band (CTB), which arises from 2the transition of 2p electrons of O to the empty 4f orbitals 3+ of Eu ions, and a group of shape lines in the longer wavelength region (310-400 nm), which arises from the f-f transi3+ 6 48 tion within the Eu 4f electron configuration. As stated above in the XRD study, there is a phase transformation to tetragonal phase from hexagonal phase as the molar ratio rises from 3:1:1 to 11:1:1. It can be seen that, the width of CTB becomes broader and the position of CTB shows obvious red shift for h-phase with respect to that for t-phase. As reported by previous studies, the CTB change is involved in the Eu-O bond length; the longer the Eu-O bond is, the longer the 9 wavelength of the CTB will be. In case of h-phase YPO4:Eu, 3+ 3+ 2− the difference of two non-equivalent Y (Eu )–O chemical



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of water molecules, which act as primary centers of nonradiative transition, trapped inside pores of hydrated h-phase 49 YPO4:Eu extended along z-axis, results in the loss of the luminescence intensity. The rate of non-radiative transition, 19 R0, is expressed as Eq. 2

R0 = A exp[-(ΔE - 2hνmax)B]

Where A and B are constants. ΔE is the energy difference 3+ between the excited and ground states of Eu ions, and its -1 value is about 10,000 - 15,000 cm . νmax is the highest available vibrational mole of the surroundings of the rare earth ion. The O-H functional group arises from H2O molecules trapped within hexagonal structure, and its stretching vibra-1 tion is about 3450 cm . Thus, the R0 value becomes large (since ΔE ≈ 2hνmax), and as a result of which, there is a significant extent of non-radiative transfer of energy from excited 3+ state of Eu ion to the surrounding phonons of O-H species, 3+ leading to a reduction in Eu luminescence intensity. (ii) As reported previously, diverse combinations between the surface and the adsorbed species induced by different shapes and sizes will produce different quenching abilities of the 50 emissions. In our work, the phase transformation also resulted in the difference in morphologies and sizes of asprepared YPO4:Eu samples, which further influences the 3+ emission intensities of Eu ions.

Figure 9. (a) Excitation spectra, (b) emission spectra, (e) the 5 7 5 7 emission intensity change of D0→ F1 and D0→ F2 transi5 3+ tions and (f) decay curves for the D0 level of Eu for YPO4:Eu samples as-prepared at different molar ratios of 3+ 33PO4 : RE : Cit . Emission spectrum of (c) hexagonal phase and (d) tetragonal phase YPO4:Eu prepared at 1:1:1 and 13:1:1 33+ 3molar ratio of PO4 : RE : Cit , respectively. 3+

Emission study. Upon excitation into the direct Eu at 395 nm, the emission spectrum (Figure 9b) of each YPO4:Eu 33+ sample (prepared in different molar ratios of PO4 : RE : 33+ Cit ) exhibits a typical emission peak of Eu at about 595 nm 5 7 corresponding to the magnetic dipole transition ( D0→ F1) along with peaks at about 617 nm and about 695 nm which 5 7 corresponds to the electric dipole transitions ( D0→ F2) and 5 7 48 ( D0→ F4), respectively. Also, weak emission peak at about 5 7 650 nm corresponding to D0→ F3 transition is observed. Apart from these, no emission from the higher energy levels 5 5 3+ ( D1, D2) of Eu is detected due to the vibration of phos-1 phate groups (ca, 1067 cm ), which can bridge the gaps be5 5 5 tween the higher energy levels ( D1, D2) and the lowest D0 3+ level of Eu effectively. 3+

It can be seen that, the emission intensity of Eu increases 3upon increasing the PO4 concentration which is being attributed to the phase transformation from hexagonal to tetragonal. The intensity of pure t-phase YPO4:Eu is about 10 times stronger than that of the h-phase YPO4:Eu. The reasons responsible for the significant difference in luminescence intensities should have two aspects: (i) the increase in luminescence intensity is mainly due to the loss of H2O on conversion to the dehydrated t-phase YPO4:Eu. The presence

Figure 9c shows the single emission spectrum of h-phase 33+ YPO4:Eu crystal prepared at 1:1:1 molar ratios of PO4 : RE : 3Cit . The emission spectrum is similar in position but unlike in transition branch to that of t-phase YPO4:Eu (Figure 9d). The spectral splitting, induced by Stark effect from different crystal structures/fields, are quite different. We can easily 3+ identify the luminescent sites of Eu within different CF surroundings by comparing the CF splitting number of the 5 7 D0→ FJ transitions with the theoretical predictions. In a 3+ hexagonal structure, Y ions sit at a D2 lattice site, whereas 3+ Y ions sit at a D2d lattice site in a tetragonal structure. The3+ 3+ oretically, if the substitution of Y with Eu ions does not induce significant lattice distortion, the three lines respec5 7 5 7 3+ tively for D0→ F1 and D0→ F2 transitions of Eu at h-phase YPO4:Eu with original D2 lattice site, and the two lines re5 7 5 7 3+ spectively for D0→ F1 and D0→ F2 transitions of Eu at tphase YPO4:Eu with original D2d lattice site should be ob5 7 served, according the allowed transition lines of D0→ FJ of 3+ 51 Eu ions at 32 crystallographic point groups. As shown in Figures 9c and d, spectral splitting numbers we observed are consistent with the theoretical prediction, indicating that the 3+ dopant Eu ions (5 at. %) induce insignificant lattice distortion for both the h- and t-phase YPO4:Eu. 3+

The asymmetric environment around the Eu in the crystal lattice can be checked by the variation of the relative in5 7 5 7 tensity ratio of the electric ( D0→ F2) to magnetic ( D0→ F1) dipole transitions. This is called asymmetric ratio (A21), relat48 ed to the Judd-ofelt parameter Ω2. A21 changes for asprepared samples at excitation (395 nm) are shown in Figure 5 7 5 7 9e. D0→ F2 transition increase compared to D0→ F1 transition along with the phase transformation to t-phase YPO4:Eu from h-phase YPO4:Eu, which lead to the increase in the value of A21. The symmetry of the crystal field will be relatively 5 7 low if the D0→ F2 transition is relatively strong since this transition is hypersensitive to its local symmetry. The in-


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Crystal Growth & Design 3+

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crease in A21 value actually suggest that Eu environment in t-phase is more asymmetric as compared to that in h-phase. 5

Lifetime study. The luminescence decay curves of the D0 3+ level of Eu have been measured for YPO4:Eu samples as33+ 3prepared with different molar ratios of PO4 : RE : Cit (Figure 9f). Excitation and emission wavelengths are fixed at 395 nm and 595 nm, respectively. The intensity increases 3with the increase in the PO4 content. Similar behavior has been observed in the case of luminescence studies. All the decay date can be well-fitted using monoexponential decay equation, It = I0 e

-t/τ

Eq. 3

Where, I0 and It are intensities at t = 0 and time t, respectively. The τ is the lifetime of the luminescence decay. The life3time increases from 1.86 to 3.85 ms with increase of the PO4 concentration, in which the phase transformation occurs, as shown in Table 2. Non-radiative transition paths, such as those involving the OH ions or the quenching centers associated within hexagonal phase YPO4:Eu, have shortened the lifetime of the excited states, and thus lowered the emission intensity. The luminescence behavior for YPO4:Eu samples prepared 3in the Cit -reduced, IPA-, EDTA- and EN-dependent experiments are shown in Figure S5, S6, S7 and S8, respectively and their lifetimes after fitting decay data with monoexponential equation are given in Table S5, S6, S7 and S8, respectively (see the Supporting Information). Similarly, the luminescence intensity and lifetime increases with the increase in 3the PO4 concentration at which the phase transformation from hexagonal to tetragonal structure takes place. Table 2 Lifetimes and crystal phases of YPO4:Eu samples 33+ 3prepared at different molar ratios of PO4 : RE : Cit 3-

3+

3-

samples

PO4 : RE : Cit (molar ratio)

crystal phase

τ (ms)

a

1:1:1

hexagonal

1.86

b

3:1:1

hexagonal

2.42

c

5:1:1


2.45

d

7:1:1


2.54

e

9:1:1


2.63

f

11:1:1

tetragonal

3.71

g

13:1:1

tetragonal

3.85

Conclusions Our study describes a YPO4:Eu crystal system in which the products crystallize with hexagonal or tetragonal structure depending on the concentrations, the phosphate groups and 3+ the free RE ions under reaction solution. The hexagonal phase is favored in the case of relatively low concentrations 3+ of the phosphate groups and the free RE ions, whereas increasing one of them in using different approaches can lead to the formation of the tetragonal phase. The hexagonal YPO4:Eu have prism-like morphology. On the contrary, the tetragonal one consists of microspheres or nanosheets. The luminescence intensity of the tetragonal YPO4:Eu is about 10 times stronger than that of the hexagonal YPO4:Eu. Also, the decay lifetime is significantly improved along with the phase transformation to tetragonal from hexagonal one. This structural control rule found here are important not only for understanding the phase formation of the YPO4:Eu crystals, but also for providing a new idea of using this rule to facile synthesis of other desirable phased nano-/microcrystals.

ASSOCIATED CONTENT Supporting Information. SEM images, excitation spectra, emission spectra and luminescence decay curves of YPO4:Eu 3samples prepared in the Cit -reduced, IPA-, EDTA- and ENdependent experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by the National Science and Technology Major Project, the National Nature Science Foundation of China (No. 61308015, 11474228), and the Key Science and Technological Innovation Team of Shaanxi Province (No. 2014KCT-10). P. Li wants to thank, in particular, the company and encouragement received from Y.X. Guo during this work.

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For Table of Contents Use Only: Manuscript title: Phase Control of Eu3+-Doped YPO4 Nano/microcrystals Authors: Peng Li, Yanpeng Zhang, Lei Zhang, Feng Li, Yaxin Guo, Yanghui Li, Weiping Gao

Synopsis: Manipulating the relative concentrations of phosphate groups and free RE3+ ions can determine the formation of final crystallographic phase of the Eu3+-doped YPO4 nano/microcrystals.


12

Microcrystals - Crystal

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